The golden hamster (Mesocricetus auratus) is being used as a possible model for the study of immunity to human kala-azar. Studies in our laboratory indicate that hamsters drug-cured of active Leishmania donovani infections or infected subcutaneously with live L. donovani parasites possess significant resistance to intracardially injected challenge infections. It is proposed to investigate various aspects of this acquired immune response to include the quantitative aspects of inoculum size and time of immunization on the ability of hamsters to control challenge infections. In addition, several species of non-human Leishmania will be studied for their ability to immunize against a human African strain of L. donovani. Finally, attempts will be made to study the mechanisms of immunity to visceral leshmaniasis to include the relationship between acquired resistance and delayed hypersensitivity to leishmanial antigens. The relative importance of cells and antibody to immunity will be also investigated using the techniques of passive and adoptive transfer and the in vitro survival of parasites in macrophages from immune versus normal hosts.